Naturally occuring phospholipid vesicles at key points in time and location undergo fusion with limiting cell membranes to release internal contents. The elucidation of the molecular factors which lead to the alteration of the stability or activity of model systems which exhibit similar phenomena is the primary goal of this work. In cases where essential molecular constituents have been identified such as the free fatty acid in a homogeneous phosphatidylcholine vesicle and the Ca ions phosphatidic acid pair in a mixed phosphatidyl cholinetidic acid vesicle, the active component is being observed spectroscopically. Experiments will employ F19, C13, H2, and nitroxide labeled lipids to characterize, inside-outside distribution, mobility, or phase composition of the active component. Results obtained will be correlated with data on the overall kinetics of the fusion process and on changes in the vesicle size distribution during fusion. Identification of essential characteristics of fusing systems should lead to the rational development of additional, useful and controllable models. Bibliographic references: H. L. Kantor and J. H. Prestegard, "Fusion of Fatty Acid Containing Lecithin Vesicles". Biochemistry, 14, 1790 (1975). M. P. N. Gent and J. H. Prestegard, "Cholesterol- Phosphatidylcholine Interactions in Vesicle Systems....", Biochemistry, 13, 4027 (1974).